The first step when fitting EyeSpace Scleral lenses is to determine the correct overall lens diameter. It is vital that the scleral lens vaults the limbal zone. Should the lens ride onto the limbal zone, it can cause long-term pathology like neovascularisation. To calculate the correct scleral lens diameter add 5.50 mm to the measured HVID. For example, if the measured HVID is 12.00 mm, then the calculation will be: 12.00 mm + 5.50 mm = 17.50 mm.
A quick and easy way to see if the calculated lens diameter is large enough is to fill the lens bowl with saline and Sodium Fluorescein (NaFl) and observe the limbal zone. With adequate clearance, the limbal zone should show bright green with the NaFl only terminating in the scleral zone where the SLZ makes contact with the sclera.
Since the cornea is oval in shape and the scleral lens normally decentres inferior-temporally, it is important to verify that the lens vaults the limbal zone in all the meridians of the eye.
If limbal clearance is not achieved, a larger diameter EyeSpace Scleral lens should be selected. When fitting EyeSpace Scleral lenses with diameters larger than 16.50 mm it is often necessary to design the lens with a toric sagittal difference (Delta SAG) of more than 150 microns. The reason why a toric haptic is needed for larger scleral lenses has to do with the anatomy of the eye.
Recent research by Ritzmann found past the 15.00 mm chord, the shape of the sclera becomes significantly more rotationally asymmetric compared to the limbal zone. They also found the average scleral toricity at the 15.00mm chord to be 150 microns, which indicate that a toric haptic will improve lens alignment for lenses that land at a chord of 15.00mm or larger.
This confirms earlier findings by Pacific University shape study showing the scleral temporal-superior angles became much flatter compared to the nasal-inferior angles as you move away from the limbus. Interestingly, they found in the scleral zone (chord length 15.00 to 20.00 mm), the difference in scleral toricity (Delta SAG) can range from 150 to 1000 microns with an average of about 400 microns.
With new scleral scanning technology like the Eaglet Eye Surface Profiler, it is easy to see the difference in nasal and temporal scleral angles.
In most cases, the increased toricity will cause the rotationally symmetric scleral lens to pivot on the flat meridian causing a rocking motion and lens decentration in the steep meridian. Please note that not all scleral shapes are the same. There are many cases where the axis of the flat meridian is not horizontal but rather vertical or even oblique.
Lens decentration can easily be observed with a cross-section view using the slit lamp. Taking corneal shape into account, in most cases downward decentration is caused by a lens pivoting over a flat horizontal meridian. Increasing the sag in the steep vertical meridian will help to centre the lens over the eye.
With horizontal decentration, the most likely cause is pterygiums that are harder to solve. In some cases, the only remedy is to let the pterygium be removed.
What confuses many is the influence a rotationally asymmetric (toric) sclera has on the Scleral Landing Zone (SLZ). In the following examples, we can see three different conditions of blanching:
In the first example below, the SLZ angle is too steep. Notice the narrow blanching at the lens edge. Typically with a tight SLZ, the blanching will not be wider than about 1.5 mm.
In the second example, the blanching is away from the lens edge. The more mid-peripheral blanching would indicate an SLZ that is too flat with the heel digging into the sclera carrying all the lens weight and the toe of the SLZ lifting away from the sclera. If this lens stays on the eye, it will lead to late afternoon hyperemia, excessive lens discomfort and difficulty to remove the lens at the end of the day.
The third example is the tricky one. At first glance, the SLZ looks too steep, but with a closer observation, the blanching is much wider than usually seen with a steep SLZ. In this example, the blanching is about 3.00 mm wide which is the full diameter of the SLZ. The full-width blanching shows that both the heel and toe of the SLZ is resting on the sclera indicating proper SLZ alignment.
In cases like these, you should suspect uncorrected (or over corrected) toricity. The meridional blanching is due to most of the lens weight depressed on only one meridian of the sclera where little or no weight is present on the opposite meridian.
You can perform some tests to confirm if indeed under or over corrected toricity is present:
When a rotationally symmetric scleral lens or a lens with insufficient toricity is placed on a high non-rotationally symmetric sclera, the one meridian of the lens will always bear down more on the sclera compared to the opposite meridian. With a slit lamp, this will typically be seen as meridional blanching.
The meridian that bears down the hardest will present with a wide blanching pattern and will become the flat meridian of the scleral lens (indicated by the two engraved lines on the EyeSpace Scleral toric lens).
The lens meridian, not bearing down on the sclera will show little or no blanching and will become the steep meridian of the EyeSpace scleral lens, requiring more sag to allow for proper lens bearing.
Perform a topography scan of the EyeSpace Scleral lens on the eye, making sure that you measure the centre portion of the scleral lens. If the scleral lens is not properly bearing on all the meridians, lens flex will occur in the steep meridian and presents as an astigmatic cylinder on the topography scan. Both the direction and magnitude of the cylinder must be recorded.
The flat K-reading will show the area of the most lens bearing.
The steep K-reading will show the area of least bearing. This will be the meridian that requires more sag to correct for the lens flex.
The amount of cylinder present in the lens over topography can be used to determine the amount of sag needed to correct the flex on the lens. For example, if the Delta-K (cylinder) was measured to be 1.00D, an amount of 200 microns must be added to the sagittal value of the steep meridian.
Insert the EyeSpace Scleral lens on the eye with clear saline and then instil NaFl. Gently depress the sclera next to the lens edge in the suspected steep meridian. If NaFl floods into the lens bowl, this would indicate that the lens is not sufficiently bearing down, and more sag should be added to this meridian.
Observe the lens with your slit lamp for any lens decentration and rocking motion. The lens will typically de-centre in the direction of the steep meridian.
A final point to remember is that the SLZ does NOT alter the sagittal lens-to-sclera relationship. By steepening or flattening the SLZ, only the angle of the SLZ is adjusted to match up with the angle of the sclera for proper edge alignment. Changing the angle of the SLZ will not correct the toricity of the fit.
The only way to compensate for scleral toricity, and eliminate lens flex, is to increase the sagittal value (z-zone) of the steep meridian.
Loose or congested conjunctiva underneath the scleral lens can have two origins:
With excessive lens bearing on the sclera, the conjunctiva tends to bunch up around the heavy bearing zone. One of the common causes is increased capillary forces sucking the scleral lens down due to excessive Post Lens Tear Thickness (PLTT) profiles in the mid-periphery of the lens. Should this be the case, the cornea-to-lens, BC and sagittal height relationship will need to change. For more information on this, please read our article How do I choose the correct EyeSpace Scleral BOZR?
In cases where the scleral lens is not bearing down on the sclera, tears will be sucked in under the lens drawing the loose conjunctiva with it. With OCT imagery, the loose conjunctiva will have a 'wave-like' appearance under the lens.
To confirm if too little lens bearing is present in the zone of the conjunctival prolapse, use the Neg NaFL staining test. Should the test be positive and NaLF flows into the lens bowl, more sag is required in that meridian to seal off the lens and stop the tear flow.
For more information on fitting EyeSpace Scleral lenses, please see the EyeSpace Advanced Scleral Guide.